Diffuse Shading not working in OpenTK

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I'm new at OpenTK and am using it with VB.Net. So far everything has worked well and performance is good, except that I can't get diffuse lighting to shade my objects (crude vehicles moving on a highway). The only light source that works is ambient. I have tried a variety of options for setting the light and materials in the Form_Load event, and the results in the attached screenshot are with the lines shown as commented out (I have tried all combinations without much difference).